The invention relates to a self-drilling screw produced from an austenitic or other stainless steel, comprising a shank having a thread-forming region and a drilling or penetrating tip.
A drilling tip of the aforementioned type is known from DE 20 2009 011 665 U1, which relates to a screw having at least a hard drilling tip and a hard threaded zone and consisting of one piece of austenitic, martensitic or ferritic stainless steel or stainless duplex steel, at least the drilling tip and the threaded zone being case hardened. Self-drilling screws of the aforementioned type have becomes accepted as economical fasteners in many fields, since they do away with the work step of pre-drilling because constructional steels with a thickness of more than 10 mm are drilled through by hardened self-drilling screws, the thread then immediately being cut and the screw fastened. Self-drilling screws which are used outdoors, that is, in the region of a building cladding, for example, where they are exposed to severe weather conditions and moisture, are additionally subject to high corrosion-resistance requirements. With self-drilling screws made of stainless V2A or V4A qualities, insufficient hardness is initially achieved in the basic structure—at least on the screw surface. Although the corrosion properties are a permanent safety factor for screw connections, screws made from stainless steel cannot be used for drilling operations in for example constructional steels without additional measures owing to their properties resulting from heat treatment.
In order to take account of market requirements, owing to this aforementioned starting point in terms of material, self-drilling screws made of high-grade steel were developed, which are or were very complex to produce or had at least one serious drawback.
A basic production principle, which is predominantly applied nowadays, involves the production of a two-part screw, the two parts of which are welded together. This is a very complex process before a drilling tip and the thread can be attached. In one configuration, the drilling tip and the thread-forming region consist of carbon steel, finished by induction- or flame-hardening for example.
To avoid these drawbacks, the aforementioned document seeks to produce a stainless high-grade steel self-drilling screw made of a single piece. This special high-grade steel self-drilling screw has at least a hard drilling tip and a hard threaded zone, which are each case hardened. It is seen from the related detailed description in this document that this type of production is too complex.
Other known methods are mentioned in this document, such as hard chromium plating, chemical nickel plating and chromising. However, these methods fell short of meeting one or more of the following essential general requirements such as suitability for mass-produced articles, both in process terms and with regard to cost aspects (technical complexity and duration of the method), environmental compatibility of the coat, technological suitability of the method for self-drilling screws in terms of the coating thickness that can be achieved, and in consideration of the strength of the base material of the self-drilling screw. In justification of the last-mentioned aspect (insufficient strength of the base material), it is stated that, despite extremely hard coatings of over 1000 HV, what is known as the “eggshell effect” occurs, whereby the coating is indented and thus becomes ineffective. The coat or coating thus cannot bring its strength properties to the application as a self-drilling screw. Lastly, aside from the self-drilling screw described as novel in the document, in which at least the drilling tip and threaded zone are case hardened, it has to date not been possible to develop a self-drilling screw which consists of a single rust-resistant material and of which the drilling tip and thread-forming region are sufficiently hard for drilling into constructional steel and are still corrosion-resistant to the required degree.
In light of this, industry still prefers rust-resistant self-drilling screws produced in two pieces and finally welded. Unfortunately, this type of self-drilling screw is complex and very costly to produce. Furthermore, it is necessary that the tip, which consists of a rusting material and is welded to the shank made of rust-resistant material, is no longer in engagement once the self-drilling screw has been set. Specifically, the drilling tip can rust away over time and thereby impair the connection produced. In addition, such known two-piece self-drilling screws are always oversized because they are only used once. On the other hand, however, a self-drilling screw of this type, which is not intended to obtain its stability through case hardening, cannot be produced from one piece, because experts unanimously feel that it is not possible to drill into steel using a fastener which consists of just one rust-resistant base material.
Although the option does indeed already exist to coat rust-resistant, single-piece self-drilling screws, according to the aforementioned document, DE 20 2009 011 665 U1, it has not yet been possible to develop such a self-drilling screw of which the drilling tip and the thread-forming region are sufficiently hard for drilling into constructional steel and are still corrosion-resistant to the necessary degree.
Abrasion- and high surface pressure-resistant hard-material coats have indeed been successfully produced on flexible substrates, as indicated by DE 10 2004 054 193 A1. However, these are clearly neither suitable nor intended for use in self-drilling screws. In addition, production of the hard-material coat is complex, since the coat has to consist of at least two morphologically different layers, the first layer, which lies directly on the substrate surface, being a metal or ceramic hard coating and the outermost layer being a hard coating of amorphous carbon.
A similar solution is offered by EP 0 761 844 B1, which discloses a method for galvanic chromium plating. This method follows a method known from another document, DE 25 02 284 C2, for the galvanic deposition of chromium coatings using a galvanic chromium plating bath. According to this last-mentioned document, glossy to metallic grey chromium coatings having a pearl-like surface with a hardness of up to approximately 1500 HV can be achieved using such a chromium plating bath. These chromium coatings with the trade name “DURALLOY” are characterised by both high wear resistance and favourable friction properties, since the homogenous spherical surface promotes wettability and thus the formation of a stable oil film. The aforementioned document EP 0 761 844 B1 aims to improve the known chromium coating to the extent that said coating has high wear resistance with low frictional values, even without lubricants. This is achieved in that the pearl-like or columnar surface of the hard chromium coating is filled up and smoothed by galvanically applied black chromium. This black chromium is applied at a coating thickness of at least 1 μm and preferably of from approximately 2 μm to approximately 6 μm. This ensures that the pearl- or column structure of the hard chromium coating is completely or largely covered. Both documents are thus clearly concerned with improving the lubrication properties and not with improving the stability, as would be necessary if such a coating were to be applied to self-drilling screws.
DE 199 13 273 C2 and DE 199 29 090 A1 do indeed respectively disclose methods for producing pearl chromium coatings on workpieces made of metal, in particular steel, and for coating a workpiece with a lubricant. However, in both cases workpieces are coated and not, for example, tools used to cut or drill such workpieces. The method according to the aforementioned document, DE 19 929 090 A1, also draws on the German patent DE 25 02 284, already cited above, in the production of a first coating. A molybdenum-sulphide-based lubricant is then applied thereto and fills up and smooths the pearl-like or columnar surface.
Furthermore, DE 101 21 593 A1 discloses a method for coating workpieces with a bearing metal. The hard chromium coating is in this case produced on a workpiece and not on a cutting tool. As a result, different parameters are important for the hard chromium coating. These parameters include the pearl-like structure being more or less regular and having a surface roughness which is at least 1 μm and up to 5 μm. The hard chromium coating is then covered with a silver coating, which smooths the hard chromium coating. The silver coating is used as a replacement for the molybdenum sulphide coating (mentioned above in a different context), which is applied in order to improve the lubrication properties. In the present case, a bearing metal, i.e. a bearing material which is subject to completely different requirements to those placed on a bit, is produced by coating the hard chromium coating with silver.
Finally, DE 32 35 447 A1 discloses a cold-forged self-drilling screw made of a rust-free austenitic steel for forming a drill portion, a thread portion and a screw head, the drill portion and the thread portion undergoing case hardening.